This invention relates to a gland sealing steam supply system for steam turbines, and more particularly to a gland sealing steam supply system for steam turbines, which is suitably used for a steam turbine in a combined cycle plant.
In a steam turbine, the leakage of the turbine driving steam to the outside is prevented by supplying sealing steam to a gland portion of the turbine, or subjecting the leakage steam from the gland portion to heat recovery, to thereby improve the operation efficiency of the turbine. The supplying of steam to the gland portion or the recovering of the leakage steam therefrom is controlled by a steam pressure regulator provided so as to be connected to a high-pressure primary steam extraction pipe branching from a high-pressure primary steam pipe through which the turbine driving seam is supplied, a pipe for high-pressure gland sealing steam connected to a high-pressure gland portion of the turbine, and a pipe for low-pressure gland sealing steam connected to a low-pressure gland portion of the turbine. During an initial period of an operation of a steam turbine, steam tends to be supplied to the high-pressure gland portion, and, in the main portion of the operation of the turbine, the steam tends to leak from the turbine. The pressure of this leakage steam is regulated by the steam pressure regulator, and the resultant steam is supplied to the low-pressure gland portion through the pipe of low-pressure gland sealing steam. When the leakage steam from the high-pressure gland portion does not serve as sufficient low-pressure gland sealing steam, supplementary steam is used, which is introduced from the high-pressure primary steam pipe to the steam pressure regulator through the high-pressure primary steam extraction pipe. When the sealing steam in the low-pressure gland portion is more than enough, the excess steam is discarded into a condenser through an additionally provided exhaust pipe extending from the pressure regulator.
The typical examples of the steam conditions for various portions of the system will now be described with reference to a combined cycle plant taken as an example. The turbine-driving inflow steam is about 57(ata) and 480(.degree. C.) during a rated operation, while the sealing steam supplied to the high-pressure gland portion by the pressure regulator is about 1.3(ata) and 450(.degree. C.). The steam obtained by regulating the leakage steam from the high-pressure gland portion by the pressure regulator and sent out to the pipe for low-pressure gland sealing steam also has steam conditions substantially identical with those for the above mentioned sealing steam. The conditions for the steam supplied to the low-pressure gland portion are determined depending upon those for the turbine driving steam discharged from the turbine, and require to be 1.3(ata) and 110.degree.-140 (.degree. C.). As is clear from the above, the pressure only may be controlled suitably on the side of the high-pressure gland portion but it is necessary to further regulate the temperature on the side of the low-pressure gland portion. The steam supplied as the sealing steam for the low-pressure gland portion to the steam pressure regulator has a sufficiently high temperature, and introducing this steam as it is to the low-pressure gland portion causes a decrease in the material values, such as thermal stress and differential expansion of a turbine rotor, i.e., produces non-preferable results. Therefore, methods of reducing the temperature of such low-pressure gland sealing steam are employed, which are disclosed in Japanese Patent Laid-Open No. 14805/1981, and which include a method of cooling the pipe for the low-pressure gland sealing steam with a primary waste gas current from the turbine before the steam has been supplied to the low pressure gland portion, or a method of cooling such a pipe with condensate from a desuperheater provided for this purpose.
Although these methods are suitably used for a regular thermal power generating turbine plant, they are not for a turbine plant for a combined cycle plant. For example, in the former method of cooling the low-pressure gland sealing steam with a waste gas current from the turbine, the pipe for the low-pressure gland sealing steam is detoured to form a loop pipe in the position in which the loop pipe faces the primary waste gas current from a rotor blade in the steam turbine, so as to improve the cooling effect. The gland sealing steam cooled with the primary waste gas current is introduced into the low-pressure gland portion through the pipe for the low-pressure gland sealing steam. However, in this method, the loop pipe is provided in a flow passage for the primary waste gas current from the turbine for the purpose of improving the steam desuperheating effect, so that the operation efficiency necessarily decreases. Especially, in a compound generating plant consisting of a gas turbine and a steam turbine using the waste heat from the gas turbine as a heat source, the capacity of the steam turbine cycle is small. Consequently, it becomes difficult to secure a space for installing the loop pipe in the gas discharge portion of the steam turbine, and installing the loop 13 in this portion of the steam turbine causes the efficiency to further decrease. In the latter method of cooling the low-pressure gland sealing steam by using an additionally-provided desuperheater, the cooling of the sealing steam is done by a desuperheater provided additionally in an intermediate portion of a pipe for the low-pressure gland sealing steam.
The condensate in a gland steam condenser, which is connected to the discharge port of a condensing pump, is parted at an outlet of the condenser and supplied to a desuperheater through a desuperheated water supply pipe, this condensate being used as cooling water. The used cooling water is returned to the condenser through a desuperheated water returning pipe. In this method, the desuperheater is provided independently on the outer or inner side of the condenser. Therefore, it is necessary that a thorough consideration be given to the designing and manufacturing of the desuperheater as a pressure vessel. Moreover, securing a space for installing the desuperheater gives rise to some problems. Especially, in a compound generating plant consisting of a plurality of units, a plurality of desuperheaters and pipes are required, so that the manufacturing cost increases.